After Kunti returned to Hastinapur, she handed over the Pandavas to Bhisma who trained them well, along with Vidura, their maternal uncle.
Duryodhana, the eldest of the Kauravas, never accepted Pandavas, as he believed that, Yudhisthira, being the elder one, would be crowned as the King.
Duryodhana despised Bhima, as he was stronger than him. In an attempt to kill him, Duryodhana, poisoned Bhima’s food, and threw him into River Ganga. Bhima was rescued by Aryaka, the Naga chieftain, who was also the maternal uncle of Bhima (Shurasena, Kunti’s father, was married to a Naga Women, Marisha) and was taken to the King of the Nagas, Vasuki.
Vasuki, impressed by Bhima’s lineage, offered him a divine mixture, Bhima obediently drank 8 pots of this mixture.
As a result, Bhima acquired the strength of thousands of Elephants. He soon returned to Hastinapur, and revealed these incidents to Yudhisthira, who asked him to remain quiet and not reveal it to Kunti.